1. Field of the Invention
This invention relates to multichannel audio and more specifically to a multichannel audio format that provides a truly discrete as well as a backward compatible mix for surround-sound, front or other discrete audio channels in cinema, home theater, or music environments.
2. Description of the Related Art
Multichannel audio has become the standard for cinema and home theater, and is gaining rapid acceptance in music, automotive, computers, gaming and other audio applications. Multichannel audio provides a surround-sound environment that greatly enhances the listening experience and the overall presentation of any audio-visual system. The earliest multichannel systems included left, right, center and surround (L, R, C, S) channels. The current standard in consumer applications is 5.1 channel audio, which splits the surround channel into left and right surround channels and adds a subwoofer channel (L, R, C, Ls, Rs, Sub).
The move from stereo to multichannel audio has been driven by a number of factors paramount among them being the consumers' desire for higher quality audio presentation. Higher quality means not only more channels but higher fidelity channels and improved separation or “discreteness” between the channels. In a truly discrete environment, discrete channels carry discrete audio signals to discrete speakers.
To satisfy this demand, the audio industry had to provide a multichannel mix from the studio or content provider, multichannel encoding/decoding techniques, a media capable of supporting multichannel audio and multichannel speaker configurations. By its very nature, multichannel audio includes significantly more data than stereo audio, which has to be compressed to fit in the existing formats and on the existing media. With the advent of media such as DVD, new formats such as 5.1 have been developed specifically for multichannel audio to enhance the listening experience.
The extension of multichannel audio beyond the 5.1 standard has once again raised the challenge of developing new encoding/decoding techniques that move the state-of-the-art forward while maintaining backward compatibility with the 5.1 standard. Having become accustomed to discrete audio, the consumer will demand the same performance as more channels are added. Backward compatibility is critical because of the great investment in 5.1 equipment by consumers and professionals alike.
Dolby Prologic™ provided one of the earliest multichannel systems. Prologic squeezes 4-channels (L, R, C, S) into 2-channels (Lt,Rt) by introducing a phase-shifted surround sound term. These 2-channels are then encoded into the existing 2-channel formats. Decoding is a two step process in which an existing decoder receives Lt,Rt and then a Prologic decoder expands Lt,Rt into L, R, C, S. Because four signals (unknowns) are carried on only two channels (equations), the Prologic decoding operation is only an approximation and cannot provide true discrete multichannel audio. As shown in FIG. 1, a studio 10 will mix several, e.g. 48, audio sources to provide a four-channel mix (L, R, C, S). (This mix may be monitored through a matrix encode and decode process.) The Prologic encoder 12 matrix encodes this mix as follows:Lt=L+0.707C+S(+90°), and  (1)Rt=R+0.707C+S(−90),  (2)which are carried on the two discrete channels, encoded into the existing two-channel format and recorded on a media 14 such as film.
A matrix decoder 16 decodes the two discrete channels Lt,Rt and expands them into four discrete reconstructed channels Lr, Rr, Cr and Sr. A passive matrix decoder decodes the audio data as follows:Lr=Lt,Rr=Rt,Cr=(Lt+Rt)/2, andSr=(Lt−Rt)/2.In general, the Lr and Rr channels have significant center and surround components and Cr and Sr have left and right components. The reproduced audio signals, although carried on discrete channels to discrete speakers in a speaker configuration 18, are not discrete, but in fact are characterized by significant crosstalk and phase distortion. For this reason passive decoders are rarely used.
Active matrix decoders reduce crosstalk and phase distortion but at best approximate a discrete audio presentation. Many different proprietary algorithms are used to perform an active decode and all are based on measuring the power of Lt+Rt, Lt−Rt, Lt and Rt to calculate gain factors Gi whereby,Lr=G1*Lt+G2*Rt Rr=G3*Lt+G4*Rt Cr=G5*Lt+G6*Rt, andSr=G7*Lt+G8*Rt. Active decode provides better compensation based on the power of the signal but crosstalk among components remains and true discrete reproduction is not possible.
The advent of the 5.1 format represented a fundamental shift in multichannel audio away from squeezing multiple channels into an existing stereo format and the phase distortion and crosstalk associated with matrix coding and to a truly discrete multichannel format, which provides higher fidelity and improved separation and directionality. Furthermore, two additional channels were added. The subwoofer (“Sub”) (0.1 channel) provides enhanced low frequency capability. The surround channel S consists of left Ls and right Rs channels indicating the consumers' strong preference for true discrete sound even in the surround channels. Each signal (L, C, R, Ls, Rs, Sub) is compressed independently and then mixed together in a 5.1 format thereby maintaining the discreteness of each signal. Dolby AC-3™, Sony SDDS™ and DTS Coherent Acoustics™ are all examples of 5.1 systems.
As shown in FIG. 2, the studio 20 provides a 5.1 channel mix. A 5.1 encoder 22 compresses each signal or channel independently, multiplexes them together and packs the audio data into a given 5.1 format, which is recorded on a suitable media 24 such as a DVD. A 5.1 decoder 26 decodes the bitstream a frame at a time by extracting the audio data, demultiplexing it into the 5.1 channels and then decompressing each channel to reproduce the signals (Lr, Rr, Cr, Lsr, Rsr, Sub). These 5.1 discrete channels, which carry the 5.1 discrete audio signals are directed to the appropriate discrete speakers in speaker configuration 28 (subwoofer not shown).
In its cinema products, DTS implemented its 5.1 system with 5 single channel APT-X encoders by taking advantage of the spectral characteristics of the surround and subwoofer channels without sacrificing performance. The use of five rather than six processors reduced system cost As shown in FIG. 3, the 5.1 signal is reformatted into a 5 channel signal with a mixer 32 that mixes the Ls, Sub and Rs into two channels using standard studio mixing techniques, i.e. the sub is reduced by 3 dB and added to the L and R surround channels. More specifically, the left and right surround channels Ls, Rs are high pass filtered, the subwoofer channel Sub is low pass filtered, and then mixed together. The Sub channel carries low frequencies and has a bandwidth less than 150 Hz and the Ls and Rs signals have only minimal low frequency content. An APT-X decoder 34 decodes the five channels and passes Lts and Rts to a demixer 36, which high pass filters them to reproduce Lrs and Rrs, and low pass filters and sums them to reproduce the subwoofer channel Sub.
Extension to discrete 6.1 and higher multichannel formats is limited by space availability on the media, reliability and the strong desire to maintain backward compatibility with existing 5.1 decoders. Multichannel audio consumes a lot of space on the medium. Providers want to extend playtime, include multiple different audio formats including 2-channel PCM, Dolby AC-3 and DTS Coherent Acoustics, add other content such as director's comments, outtakes, etc.
Dolby has developed Dolby EX, as described in PCT Publication W099/57941, which provides more than two surround-sound channels in the current 5.1 formats and does so without increasing space requirements (number of bits or film space). Dolby EX provides more than two surround sound channels within the format of a digital soundtrack system designed to provide only two surround sound channels. Three main channels are recorded in the discrete soundtrack channels and 3, 4 or 5 surround-sound channels are matrix-encoded and recorded in two discrete surround-sound soundtrack channels. The digital audio stream of the digital soundtrack system designed to provide only two surround sound channels remains unaltered, thus providing compatibility with existing playback equipment. Moreover, the format of the media carrying the digital sound tracks is unaltered. Dolby asserts that the “discreteness” of the digital soundtrack system is not audibly diminished by employing matrix technology to surround sound channels, particularly if active matrix decoding is employed.
Dolby EX introduces phase-shifted surround sound terms to matrix encode the 3, 4 or 5 surround-sound signals into two channels, which facilitates decoding the two channels into 3, 4 or 5 audio channels. The introduction of the phase-shifted terms is essential to Dolby EX as it was to Dolby Prologic. The encoding process is given by the following generalized equations:Lts=Ls+ΣGi*Si(φi) for i=0, 1, 2, andRts=Rs+ΣHi*Si(−φi) for i=0, 1, 2where Gi and Hi are the gain coefficients, Si are the additional surround-sound channels and φi are the phase distortion components. The decoding process is given by the following generalized equations:Lrs=G1*Lts+G2*Rts Rrs=G3*Lts+G4*Rts Crs=G5*Lts+G6*Rts In the special case of three surround-sound channels (Ls, Rs, Cs), these generalized equations default to the well known mix equations where the Cs channel is reduced by 3 dB and added to the Ls and Rs channels as follows:Lts=Ls+0.707Cs, andRts=Rs+0.707Cs. It is believed that actual Dolby Ex systems phase shift Ls and Rs by plus and minus 45 degrees, respectively, to provide more depth to the surround sound. The QS or SQ matrix systems cited in the PCT Publication teach that technique.
As shown in FIG. 4, in a Dolby Ex system 40 the studio 42 provides a 6.1 channel mix (L, R, C, Ls, Rs, Cs, Sub) where Cs is an additional center surround channel. A matrix encoder 44 applies the Prologic coding algorithm to the three surround sound channels (Ls, Cs, Rs) to matrix encode them into Lts and Rts. The 5.1 channels L, R, C, sub, Lts, Rts are encoded using an AC-3, Sony or DTS encoder 46 and recorded onto a media 48. A 5.1 decoder 50 decodes the audio data to reproduce the discrete L, R, C and Sub audio channels and pass the matrix encoded Lts and Rts channels to a matrix decoder 52, which matrix decodes the channels into Lrs, Crs and Rrs using the same active matrix techniques as the Pro Logic decoders. The 6.1 discrete channels are directed to discrete speakers 54 for audio playback.
It is important to note that the three discrete surround channels do NOT carry discrete signals. The same crosstalk and phase distortion limitations associated with Prologic are now reintroduced into what was a truly discrete multichannel system. While it is true that a listener's sensitivity to position and direction is less for rear signals, true discrete audio reproduction will provide better sound separation and directionality. For the same reasons consumers preferred 2-channel surround over mono surround they will prefer 3-channel discrete surround over matrixed 2 channel surround.
Dolby EX represents a first step toward enhanced multichannel audio. Dolby EX provides additional surround sound channels using existing 5.1 formats without increasing the bit rate. Furthermore, Dolby EX preserves the discrete coding of L, R, C and sub audio signals. However, Dolby EX achieves these desirable results by sacrificing the true discreteness of the surround sound channels. A 3:2:3 system will suffer the same crosstalk limitation as Pro Logic. 4:2:4 and greater systems will also suffer phase distortion problems due to the matrix decode.
Dolby cannot provide true discrete N.1 audio because audio quality and/or reliability will suffer. The PCT Publication contemplates and then dismisses a new N.1 format for truly discrete audio stating “Although, in theory, additional channels could be carried by reducing the symbol size in order to provide more bits and allowing the storage of more data in the same physical area, such a reduction would introduce unwanted difficulties in the printing process and require substantial modification or recorder and player units in the field.” A true N.1 format would be incompatible with existing hardware and would require at least substantial modification if not total replacement.
Accordingly, there remains an unfulfilled need in the industry to provide a truly discrete multichannel surround sound environment with more than two surround channels while maintaining backward compatibility with existing 5.1 decoders without sacrificing audio quality or reliability.